Engine ignition systems may include a spark plug for delivering an electric current to a combustion chamber of a spark-ignited engine to ignite an air-fuel mixture and initiate combustion. Based on engine operating conditions, spark plug fouling can occur wherein a firing tip of the spark plug becomes coated with a foreign substance, such as fuel, oil, or soot. Once fouled, the spark plug may be unable to provide adequate voltage to trigger cylinder combustion until the spark plug is sufficiently cleaned or replaced. For example, the spark plug may be cleaned by operating the engine in speed-load conditions that sufficiently raise the spark plug tip temperature, thereby burning off the accumulated soot. However, high spark plug tip temperatures may result in the pre-ignition of fuel. Avoiding pre-ignition is made more difficult in engines that inject washer fluid to improve knock limits and fuel economy. In particular, methanol contained within washer fluid has a lower surface ignition temperature than gasoline, making methanol more prone to pre-ignition than gasoline if spark plug tip temperatures are too high. As such, it may be difficult to provide a spark plug having a heat range that is hot enough at light loads to burn off accumulated soot, but cool enough at high loads to avoid pre-ignition.
One example approach for mitigating spark plug fouling and pre-ignition is shown by Russell et al. in U.S. Pat. No. 7,886,729. Therein an engine system includes two spark plugs in an engine cylinder, each spark plug operating at different heat ranges in order to avoid misfire and pre-ignition. Responsive to a combustion chamber temperature, the spark plugs are selectively operated above a fouling range and below a pre-ignition range specific to each spark plug. The fouling and pre-ignition ranges of each spark plug are selected so that one spark plug may be operated at higher temperatures and the other spark plug may be operated at lower temperatures, reducing the likelihood of misfire due to spark plug fouling and pre-ignition due to high spark plug tip temperatures.
However, the inventors herein have recognized potential issues with such systems. As one example, the fouling ranges of both spark plugs may overlap over a range of combustion chamber temperatures. That is, at certain combustion chamber temperatures neither spark plug may be able to provide reliable ignition. To address this issue, an engine controller may quickly transition between operating conditions to avoid operating at these temperatures, but a quick change in engine operating conditions may cause torque disturbances and associated issues. On the other hand, if the controller does not quickly transition between acceptable operating conditions and operates in the fouling range of both spark plugs, the spark plugs may become soot fouled, leading to cylinder misfire events. In addition, since this engine system utilizes two spark plugs in a combustion chamber, component cost and engine complexity is increased. Further, pre-ignition may be induced by the presence of carbon deposits in the combustion chamber.
In one example, the issues described above may be addressed by a method for an engine comprising, in response to a torque reduction request received while a predicted engine soot load is higher than a threshold soot load, injecting a fluid into an intake manifold, and adjusting spark ignition timing based on the fluid injection. The fluid may include water or washer fluid. In this way, the fluid may be used to decarbonize the fouled spark plug and limit the occurrence of engine misfires and also decarbonize deposits from a combustion chamber to limit knock and pre-ignition incidence.
As one example, an engine may be configured with a low heat range spark plug to reduce incidence of pre-ignition at high engine speed and load conditions. As such, the low heat range spark plug may not get warm enough to burn off soot deposits from a fouled spark plug. Therefore during conditions when the spark plug needs to be cleaned (such as when the spark plug soot load is higher than a threshold), and/or when the combustion chamber needs to be cleaned (such as when the combustion chamber soot load is higher than a threshold), an amount of washer fluid (including water and methanol) may be injected to decarbonize the spark plug and the combustion chamber. Since the washer fluid injection slows the combustion rate, knock and pre-ignition are reduced, allowing the engine to be operated with more spark advance. Thus while injecting the fluid, spark timing may be maintained or advanced. By advancing the spark timing, more heat is retained in the combustion chamber, thereby increasing the heat transferred to the spark plug and the combustion chamber, burning off the accumulated soot deposits. The engine controller may also determine if there is an upcoming torque reduction request, such as due to a transmission gearshift. If so, the fluid injection can be performed opportunistically during the transmission shift, enabling a reduction in engine torque output and a concurrent cleaning of the soot load. The amount of fluid injected may be adjusted based on the amount of soot load on the spark plug or the combustion chamber, a likelihood of abnormal combustion due to pre-ignition, knock, or misfire, and where the injection is performed during a torque reduction, the desired torque reduction. The amount of fluid injected may be increased as the spark plug or combustion chamber soot load increases, and/or as the desired torque reduction increases. By injecting the fluid during the transmission shift, the engine can be decarbonized while the need for spark retard during the torque reduction is decreased, improving fuel economy. In some examples, in addition to adjusting the amount of washer fluid injected, a rate of the injection may be adjusted based on the soot load, desired torque reduction amount, likelihood of pre-ignition, knock incidence, etc. For example, higher rates of injection may be used to decarbonize the spark plug and combustion chamber at higher soot loads. The amount and rate of washer fluid injection may also be adjusted based on the methanol content of the washer fluid to account for the increased propensity of methanol for pre-ignition (due to its lower surface ignition temperature). While the above example discusses the injection of washer fluid, in alternate examples, the soot load may be addressed by injecting water.
In this way, water or washer fluid injection may be used to reduce spark plug soot fouling and limit misfire occurrence. In addition, carbon deposits can be cleaned from the combustion chamber, limiting pre-ignition occurrence. The technical effect of injecting water or washer fluid during a torque reduction event is that engine torque output may be lowered with reduced need for spark retard while a spark plug and combustion chamber are concurrently cleaned. In particular, the injected water or methanol absorbs combustion heat, slowing the combustion rate. Due to the slowed rate of combustion, torque delivery is reduced, thereby reducing spark retard usage and increasing fuel economy. At the same time, the injected water (and methanol) may vaporize to form steam that may clean the spark plug and the combustion chamber. The injection of washer fluid or water also allows the engine to be operated with more spark advance, allowing for more heat to be retained in the cylinder, improving decarbonization efficiency. By cleaning the spark plug and combustion chamber deposits, abnormal combustion due to misfire, knocking, and pre-ignition is reduced. In addition, spark plug life is extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.